OBJECTIVE: To investigate aquaporin 9 (AQP9) mRNA and protein expression in antrum follicle and luteinizing granulosa cells of polycystic ovarian syndrome (PCOS) ovary, and its relation to follicular fluid steroids hormone levels during IVF cycles. METHODS: AQP9 mRNA expression on luteinizing granulosa cells in IVF cycles was detected by RT-PCR. AQP9 protein expression in antrum follicles of PCOS ovary and luteinizing granulosa cells was measured by immunohistochemistry. The concentrations of estradiol (E2), progesterone (P) and testerone (T) in follicular fluid were measured by radioimmunoassay (RIA). RESULT: The expression of AQP9 mRNA in luteinizing granulosa cells during IVF cycles was positive by RT-PCR. No significant differences in AQP9 mRNA levels in granulosa cells between PCOS and control group were found during IVF cycles. The expression level of AQP9 mRNA in large follicles was higher than that in small follicles, but not significantly. The immunoreactivity for AQP9 was localized in membrane and cytoplast of granulosa cells in antrum follicles from PCOS ovary and luteinizing granulosa cells during IVF cycles. Multiple regression analysis showed that AQP9 mRNA levels on granulosa cells were not correlated with E2, P and T levels in follicular fluid during IVF cycles. CONCLUSION: AQP9 may play an important role in the follicle development and antrum formation through water transport and AQP9 may be involved in the mechanism of follicle development in PCOS.